I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to determining the position of a mobile radiotelephone using the time of arrival estimates for GPS and CDMA pilot signals.
II. Description of the Related Art
The Global Positioning System (GPS) is a space based, radio-positioning and time transfer system. The system was originally developed primarily for military purposes but is now used extensively in civilian applications as well.
GPS provides accurate position, velocity, and time information for a given object anywhere on the earth. Twenty-four GPS satellites, arranged in six orbital planes, transmit radio frequency ranging codes and navigation data messages. The navigation messages include such data as satellite clock-bias data, ephemeris data, certain correction data, and the course orbital data on the twenty-four satellites.
Generally, the satellites transmit ranging signals on two D-band frequencies. The satellite signals are transmitted using spread-spectrum techniques, employing ranging codes as spreading functions. These spreading functions consist of a 1.023 MHz coarse acquisition (C/A) code and a 10.23 MHz precision code. The codes are designed to minimize the probability that a receiver will mistake one code for another (i.e., minimize cross correlation).
The ranging codes broadcast by the satellites enable the GPS receiver to measure the transit time of the signal and thereby determine the range between the satellite and the receiver. Typically, four GPS satellites must be in clear view of the receiver in order for the receiver to accurately determine its location. The measurements from three GPS satellites allow the GPS receiver to calculate the three unknown parameters representing it's three-dimensional position. The fourth satellite allows the GPS receiver to calculate the user clock error that is generally unknown.
A common problem with the conventional GPS is not having four GPS satellites in clear view of the GPS receiver. This typically arises in a city setting in the shadow of a group of tall buildings. In such situations, the GPS receiver is unable to accurately determine its location using GPS.
Additionally, even when four satellites are in view, there are further errors that result in erroneous position determinations. These errors include physical errors such as signal path delays through the atmosphere (i.e., propagation signal delay and satellite clock and ephemeris errors). Also, the government introduces errors for national security reasons that include ephemeris data error and clock error.
To reduce the effects of these errors, a differential GPS (DGPS) system may be used. Typical DGPS architecture includes one or more reference stations at precisely known, fixed reference sites, and DGPS receivers. The reference station includes a reference receiver antenna, a differential correction processing system, and data link equipment.
There are two primary variations of the differential measuring techniques. Both techniques are well known in the art.
The typical DGPS presents certain drawbacks. One is that the DGPS must use it own frequency band so as not to interfere with that of the stand alone GPS. In addition, the DGPS receiver presents an additional receiver that must operate independently of the GPS receiver in receiving the differential correction data. This goes against the industry trend to make electronics smaller and cheaper.
The present invention is also related to cellular technology. A typical cellular network is comprised of a number of cells covering a geographical area. Each cell has a base station that maintains communication with a mobile or stationary radiotelephone. The base station includes a transmitter, a receiver, and an antenna that transmits a wireless signal over the area. The transmit power of the base station is directly related to the size of the cell such that the larger the cell, the greater the transmit power of the base station.
The overall management of the cellular system is handled by a mobile telecommunications switching office (MTSO). The MTSO provides numerous functions for the cellular system, such as assigning calls to a cell based on availability and signal strength, call statistics, and billing for the network. The MTSO also functions as the interface between the cells and the public switched telephone network (PSTN).
The cellular base stations receive the GPS clock signals. Using this information, all the base stations within a geographical area are synchronized together since they are all locked to the common GPS clock.
In a code division multiple access (CDMA) cellular system, each base station transmits a pilot signal at all times. The pilot signal tells the radiotelephones which base station they are receiving and synchronizes the radiotelephones with that base station. The CDMA system is well known in the art.
The United States Federal Communication Commission is requiring that all mobile network service providers add a mobile caller location feature by October 2001. This feature should be able to locate the mobile with an accuracy of 125 meters RMS throughout their coverage area.
FIG. 1 illustrates a prior art method for determining the location of a mobile radiotelephone (101) using a GPS satellite (105). The figure shows a mobile radiotelephone (101) comprising a DGPS receiver (102) that receives the GPS satellite signal. The mobile radiotelephone (101) also has a CDMA receiver (103) for receiving a pilot signal from the CDMA base station (110).
The base station (110) is comprised of a GPS receiver (111) for receiving the synchronizing clock signal from the GPS satellite (105). The base station (110) also has a transmitter (112) for transmitting the pilot signal to the mobile radiotelephone (101).
Referring to FIG. 1, the measured time of arrival (TOA) of the i.sup.th GPS signal is denoted as T.sub.m,gps,i and is given by: EQU T.sub.m,gps,i =T.sub.sat,m,i +T.sub.rx,gps +T.sub.clk,gps
where:
T.sub.sat,m,i is the differential propagation time between the i.sup.th GPS satellite, the serving base station and the mobile radiotelephone. It depends only on the position of the radiotelephone relative to the serving base station. The satellite position is assumed to be sufficiently accurately known via the navigation data from the ephemeris data decoded at the base station;
T.sub.rx,gps is the unknown delay through the radiotelephone receiver for the GPS signals; and
T.sub.clk,gps is the clock error or the radiotelephone for the GPS signals. Likewise, the TOA of the i.sup.th CDMA pilot signal is denoted by T.sub.m,cdma,i and is given by: EQU T.sub.m,cdma,i =T.sub.b,m,i.+T.sub.tx,cdma,i +T.sub.rx,cdma +T.sub.clk,cdma
where:
T.sub.b,m,i is the propagation time between the i.sup.th base station and the mobile.
T.sub.tx,cdma,i is the unknown delay through the i.sup.th base station transmitter for the CDMA signals; PA1 T.sub.rx,cdma is the unknown delay through the mobile receiver for the CDMA signals; and PA1 T.sub.clk,cdma is the clock error for the mobile for the CDMA signals.
If only the DGPS signals are considered, the unknowns are T.sub.sat,m,i, T.sub.rx,gps, and T.sub.clk,gps. Since T.sub.rx,gps, and T.sub.clk,gps are not separable, T.sub.rx,gps +T.sub.clk,gps is treated as a single unknown. T.sub.sat,m,i is dependent on the {x, y, z} coordinates of the radiotelephone. Hence, there are a total of four unknowns to be found, thus requiring four satellites be in view simultaneously.
The CDMA pilot signals are considered next. The unknowns are T.sub.b,m,i, T.sub.tx,cdma,i, and (T.sub.rx,cdma +T.sub.cIk,cdma) The difference between the CDMA pilot signals and the GPS signals is that we have an additional unknown for each base station transmitter (i.e., T.sub.tx,cdma,i).
The problem with the location process of FIG. 1 is that there is no practical method for calibrating the unknown variables T.sub.tx,cdma,i and T.sub.rx,cdma. Therefore, the TOA measurements of the pilots cannot be directly used to augment the DGPS solution. Additionally, since the GPS and CDMA signals are processed through different channels in the radiotelephone, errors occur due to the separate channels drifting randomly with time and temperature. There is a resulting need to calibrate and compensate for these unknowns in order to provide a more accurate location mechanism.